Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa (1,100 psia) and temperatures from about 20.degree. C. to about 40.degree. C. Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is -82.5.degree. C. This means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is typically between about -85.degree. C. and -62.degree. C. Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about -165.degree. C. and -155.degree. C. Since refrigeration equipment represents such a significant part of the LNG facility cost, considerable effort has been made to reduce refrigeration costs.
Many systems exist in the prior art for the liquefaction of natural gas by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the gas liquefies. Conventional liquefaction cools the gas to a temperature of about -160.degree. C. at or near atmospheric pressure. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane. Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) "cascade cycle" which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature, (2) "expander cycle" which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature, and (3) "multi-component refrigeration cycle" which uses a multi-component refrigerant in specially designed exchangers. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
One proposal for reducing refrigeration costs is to produce liquefied natural gas at temperatures above -112.degree. C. (-170.degree. F.) and a pressure sufficient for the liquid to be at or below its bubble point. This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG which is at or near atmospheric pressure. PLNG requires significantly less refrigeration since PLNG can be more than 50.degree. C. warmer than conventional LNG. For most natural gas compositions, the pressure of the PLNG ranges between about 1,380 kPa (200 psia) and about 3,450 kPa (500 psia). In the storage, transportation, and handling of PLNG, there can be a considerable amount of "boil-off." A needs exist for a process for re-liquefying PLNG boil-off gas to again produce PLNG and at the same time having power requirements that are economic.